doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2.75" shells in 2.5" chambers

Rocketman:
Very good post & sure throws some Meat into the discussion. One question, in your opinion what does cause the 900 psi pressure rise of the Fig 8 over the RXP12. That is about an 9.2% increase. Certainly the difference in the wts of the two wads would not account for a 9.2% increase in total ejecta wt.

I found some published coefficients for teflon on steel. They showed .04 for dry condition. Polyethylene HDPE is what I believe is most of the wads are today. It's pretty slippery stuff and self releases from molds, which is why it's so popular for so many products. Tensile runs around 15 ksi. Some of the really tough steel type wads are nylon or other and can be very high tensile strength. Kinda gives a swag, but Miller's right; only an appropriate test would give the answer to any degree of accuracy.

But the notion that acceleration of the payload would not roughly pace with pressure due to differences in wad friction, when comparing significantly different pressures in loads otherwise the same, has little supporting it.

These are quick and simple calculations. I didn't include weight as I don't think it factors into this part of the problem. I wasn't trying for total friction force, only the numbers if the wad generated sufficient friction to block movement; one very special case. Forcing cone friction should closely follow the barrel bore. Chuck gave us some real numbers for plastics closely related to wad materials. Also, the wad will quickly heat from the friction and powder gas. My guess is that friction will go way down due to surface lubricity as the wad heats.

2-p, my guess would be that the exact shape of the initial combustion chamber would be important and the exact fit of the wad against the hull wall. If the wad had a somewhat loose fit, there could be a bit of initial blow-by. If the wad had a very tight force fit to the hull wall, there could be some initial resistance to movement above the wad-shot weight. We know that peak chamber pressure is pretty sensitive, so these factors seem plausible to me.

Chuck, thanks for the Mu values. I'd guess the HDPE is close to teflon in Mu. And, I surely agree that we need some accelerometer or high respones load cell force data!!

Fluid friction coefficients as well as all other friction coeffs can't be derived or calculated and are empirical in nature so a guess would be a start but would have to be validated. Since we are dealing with a fluid event, there are going to be forces tangent to the wad surface as well as perpendicular. What would the coeff. of friction be for teflon on teflon and would the event realize peak pressure during the teflon-teflon friction?

Kind Regards,

Raimey
rse

I think wads and hulls are close relatives of HDPE - pretty slick stuff. The peak pressure usually occurs within 1/4" to 3/4" of displacement, so the wad is still generally within the hull.

If the wad is still in the hull, then the length of the shell would make no real difference.

Pete

I agree that the differences should be relatively small - say as found by Bell. I imagine that the hull mouth opening into the cone does add a small amount of resistance to wad movement, just enough to bump peak pressure a few %. Burning pressure is a big factor in nitro powder burn rate. As a consequence, nitro powder gets increasingly touchy as pressure goes up. Pressure too low and it doesn't burn well. Pressure too high and it becomes intolerant of small variations in burn factors.

I've seen one opinion that smokeless needed to achieve a pressure around 7k to perform; is that close, fact or fiction or does if vary from powder to powder? Are most shotshell powders flake?

Kind Regards,

Raimey
rse